Process and apparatus for drying porous material

ABSTRACT

Porous material, for example, spongy polyvinyl acetal article, containing volatile liquid such as water is uniformly dried by charging the porous material into a closed drying chamber, blowing drying air, preferably, of a temperature higher than room temperature onto the porous material, and directing microwaves of very high or ultra high frequency onto the porous material without deterioration in quality.

United States Patent Koide et a1.

[ PROCESS AND APPARATUS FOR DRYING POROUS MATERIAL [75] Inventors: TohruKoide, Yao; Takahiro Hattori, Okazaki; Yoshio Ohno; Mitsuru Maruya, bothof Sowamachi, all of Japan [73] Assignee: Kanebo Ltd., Tokyo, Japan [22]Filed: Mar. 16, 1973 [21] Appl. No.: 341,899

[30] Foreign Application Priority Data Apr. 11, 1972 Japan 47-36245 [52]U.S. Cl. 34/1, 219/1055 [51] Int. Cl. F26b 3/34 [58] Field of Search34/1; 219/1055 [56] References Cited UNITED STATES PATENTS 3,434,2203/1969 Forster 219/1055 1 Jan.7, 1975 Primary ExaminerKenneth W. SpragueAssistant Examiner-Larry l. Schwartz Attorney, Agent, or Firm-Armstrong,Nikaido & Wegner [57] ABSTRACT Porous material, for example, spongypolyvinyl acetal article, containing volatile liquid such as water isuniformly dried by charging the porous material into a closed dryingchamber, blowing drying air, preferably, of a temperature higher thanroom temperature onto the porous material, and directing microwaves ofvery high or ultra high frequency onto the porous material withoutdeterioration in quality.

7 Claims, 10 Drawing Figures MOISTURE CONTENT mama Jan. 7,. 1915 5Shoats-Sheet 1 Timu 16 (I? I8 TIME (HOUR) I Fig. 2

Patented Jan. 7, 1975 5 Sheets-Sheet 2 VA w. Q O VA v I i Patented Jan.7, 1975 I 3,858,329

5 Sheets-Sheet 5 Fig 9 PROCESS AND APPARATUS FOR DRYING POROUS MATERIALThe present invention relates to a process and apparatus for dryingporous materials, particularly, relates to a process and apparatus foruniformly drying porous material containing volatile liquid such aswater within a short time without deterioration in quality of the porousmaterial.

Broadly speaking, it is difficult to uniformly quickly dry a porousmaterial having numerous continuous pores containing therein volatileliquid by the conventional heat-drying process. For instance, polyvinylacetal porous articles are usable as filter material and scrubbingmaterial due to numerous continuous fine pores capable of containing alarge amount of liquid such as water. Such porous articles are producedby the process wherein a mixture of polyvinyl alcohol, starch as apre-forming agent, aldehyde compound and sulfuric acid as a reactioncatalyst in water is heated to convert polyvinyl alcohol to polyvinylacetal. Usually, by the process, 60 to 85 percent by mol of hydroxylgroups in the polyvinyl alcohol are converted to acetal groups. Theproduct of the reaction is insoluble in water and has numberlesscontinuous pores of a size of several to several hundred micron and aporosity of 70 to 95 percent. The product thus produced is washed anddried. The resultant polyvinyl acetal porous article has a highhydrophilic property derived from the hydroxyl groups of thenon-converted polyvinyl alcohol and a high resistance to water andchemicals derived from the acetal groups of the polyvinyl acetal. In theprocess for producing the polyvinyl acetal porous article, the reactionproduct is washed and then dried by the conventional drying methodwherein a high temperature heating medium such as hot air, comes intocontact with the porous article. However, it is difficult to completethe drying within a short time. The reason for the difficulty willbecome apparent by reading the following description.

In the conventional heat-drying method, water located in the outermostportion of the porous article is evaporated by being imparted latentenergy for vaporization from the heating medium, and then, water locatedin the inner portion of the porous article migrates to the outermostportion so as to make the distribution of water in the porous articleuniform throughout. The migrated water comes into contact with theheating medium, and evaporates. By the above-stated proceedings, thewater in the porous material is successively evaporated.

However, due to the high hydrophilic property of the hydroxyl groupsremaining in the polyvinyl acetal porous article, a portion of water ismaintained in the inner portion of the porous article so as to resistthe migration and evaporation during drying. In order to forciblymigrate and evaporate the maintained water, it is necessary to raise thetemperature of the inner portion.

However, owing to a relatively high specific heat of water and lowheat-conductivity of the polyvinyl acetal, the velocity of heatconduction through the porous article is very low. Further the rapidevaporation of water located in the outermost portion resultsinshrinkage of the outermost portion. This shrinkage obstructs themigration of water from the inner portion to the outermost portion. Thatis, in the conventional heat-drying the polyvinyl acetal porous articlecan be dried rapidly only in the earlier stage of drying, but the dryingvelocity decreases rapidly with the lapse of time of drying in latterstage. Additionally, the conventional heat-drying process tends to anon-uniform distribution of water in the porous article. This results innon-uniform quality of the dried article, for example, in porosity andsize of pore.

It is well-known that electromagnetic microwaves of very high frequency(VHF) and ultra high frequency (UHF) are utilizable as a heating mediumfor cooking and welding. In this case, the subject matter of the cookingor welding is heated to a high temperature within a short time. However,the microwaves are not practical for use in drying the porous materialsuch as the polyvinyl acetal porous articles, because theelectromagnetic energy imparted to the porous material is consumed notonly to evaporate the volatile liquid such as water, but to raise theporous material itself to a high temperature up to the boiling point ofwater, C at which the porous material is deteriorated in quality.

Further, it is known that in the drying or heating method usingmicrowaves, it is difficult to accurately control the temperature of theporous material lying under the radiation of microwaves.

Particularly, in the case of the polyvinyl acetal porous article, themicrowave drying process tends to change in quality, because thepolyvinyl acetal has a high plasticity and a low resistance to hotwater. That is, if quickly dried using the microwave, the polyvinylacetal porous articles often tend to partly fuse or dissolve in watercontained in the porous article itself. This causes a change of theporosity, size, configuation and number of the pores. It should be notedthat even if the microwaves are imparted with a very small amount ofenergy, the above-mentioned disadvantages generally occur. Also, suchradiation ofa small amount of microwave energy results in a very lowefficiency of drying of the porous material. In order to eliminate theabove-stated disadvantages, periodical radiation of the microwaves hasbeen attempted. However, such attempts have resulted in failure becauseof difficulty in controlling the temperature of the porous material tobe dried.

The inventors, as a result of long study, have found the facts asdetailed below. In the case where the microwaves are imparted to theporous material to dry it, the vapor of the volatile liquid such aswater covers the surface of the porous material under a high partialpressure so as to obstruct the sequent evaporation of the volatileliquid, and the porous material is raised to a high temperature. Inorder to protect the porous material from the elevation of temperature,it is necessary to enhance the velocity of evaporation of the volatileliquid. This enhancement is accomplished by promoting diffusion of thevapor around the surface of the porous material and lowering partialpressure of the aqueous vapor in the porous material. Such promotion canbe accomplished by blowing air onto the surface of the porous materialso as to blow away the vapor covering the surface. That is, it wasdiscovered that by blowing air onto the surface of the porous material,almost all the imparted microwave energy can be used in evaporating thevolatile liquid in the porous material quickly, and therefore, theporous material can be uniformly dried with a practically no elevationof temperature of the porous material itself.

The present invention was completed on the ground of the abovediscovery.

An object of the present invention is to provide a process and apparatusfor uniformly drying porous material containing a volatile liquidtherein within a short time.

Another object of the present invention is to provide a process andapparatus for quickly drying a porous material containing volatileliquid therein without change in quality of the porous material.

The above objects can be accomplished by the process and apparatus ofthe present invention.

According to the process of the present invention, air is blown onto aporous material containing volatile liquid therein, for example, apolyvinyl acetal porous articles containing water therein, preferably,in at least the later stage of the air blowing, electromagneticmicrowaves are imparted to the porous material, whereby the porousmaterial is uniformly dried without change in quality thereof.

Also, according to the present invention, the apparatus for dryingporous material containing volatile liquid such as water thereincomprises a closed drying chamber, for containing porous material to bedried, means for blowing drying air into the drying chamber and meansfor directing electromagnetic microwaves into the drying chamber.

The objects, features and advantages of the present invention willbecome apparent by reading the following description while referring tothe accompanying drawings, wherein;

FIG. 1 is a diagram showing relationship between moisture content ofporous material and drying time,

FIG. 2 is a schematic view of an embodiment of the apparatus of thepresent invention,

FIG. 3 is a schematic view of another embodiment of the apparatus of thepresent invention,

FIG. 4 is a schematic plane view of further embodiment of the apparatusof the present invention,

FIG. 5 is a schematic plane view of a part of the apparatus of FIG. 4showing a method of radiating microwaves onto carriages containing aporous material of large volume, or an accumulation of numerous smallamounts of porous material, I

FIG. 6 is a schematic side view of a part of the apparatus of FIG. 4showing another radiation method of microwaves onto trucks containing aporous material of large volume,

FIG. 7 is a schematic plane view of a still another embodiment of theapparatus of the present invention,

FIG. 8 is a schematic side view of a truck of the apparatus of FIG. 7showing a circulating mechanism for a plurality of porous materials,

FIG. 9 is a schematic front view of still another embodiment of theapparatus of the present invention having a douser plate for preventingleakage of microwaves, and

FIG. 10 is a schematic side view of a conveyer chain containing guttercapable of preventing leakage of microwaves therethrough.

Referring to FIG. 1, curve A shows a relationship between drying timeand the amount of water contained in the porous material when the porousmaterial is dried by the conventional hot air drying method. It isobvious that curve A approaches an equilibrium percentage of water inthe porous material in the form of a hyperbola and thus, the hot airdrying requires a long time to complete. In such drying procedure, thereis large difference in the amount of aqueous vapor between the outermostportion and the inner portion of the porous material. Therefore, a largedifferential pressure is produced between the outermost and innerportions. The differential pressure causes nonuniformity of the driedporous material in quality, for example, porosity and size of pores.

The hot air drying of the porous material is carried out in accordancewith the following Equation 1:

(1) wherein W is a percentage of water with respect to the weight of theporous material during drying, W is a percentage of water initiallycontained in the porous material, W,, is a percentage of water in theequilibrium condition, T is a time constant which refers to a time longenough for the porous material to have reached the equilibrium conditionif the drying had proceeded at the initial drying rate, t is a dryingtime, and e is the base of a natural logarithm. From Equation 1, it isevident that the value of W varies as a negative exponential functionoft, and therefore, a long time is necessary to complete the drying.Generally, provided W -W the value of t is as large as about four timesthat of T which is a time constant.

When the porous material is dried using the hot air and theelectromagnetic microwaves, the relationship between W and r is shown byEquation 2:

wherein W, W,,, W t, and T are as defined above, and K is a capacity ofdrying by the electromagnetic microwaves. That is, Equation 2 includesaprimary proportional factor K: withrespect to t. This indicates that inthe drying process using the hot air and electromagnetic microwaves, theelectromagnetic energy imparted is consumed at an early short stage ofdrying for elevating the temperature of the porous material and volatileliquid, and thereafter, for evaporating the volatile liquid. I

A drying capacity dw/dr of the porous material at an early stage ofdrying hot air is defined by Equation 3:

When the drying capacity of the microwave. drying for the porousmaterial is m times that of the hot air drying capacity, Equation 2 ismodified to Equation 4:

From Equation 4, provided m t= 0.85T, t is the time necessary for theamount of water in the porous material to reach the equilibriumcondition, that is, the time necessary for the value (e m t/T) to reach0.

That is, when electromagnetic microwave radiation is utilized with adrying capacity of one-half times that of hot air, the drying timebecomes about one-fifth that of the hot air.

In order to shorten the drying time to one-half of that of the hot air,it is necessary that electromagnetic microwave radiation, with a dryingcapacity of 0.07 times that of the hot air, be added to the hot airdrying.

The amount of water evaporated by the microwave radiation is as follows:

Also, the amount of water evaporated by the hot air is as follows:

Therefore, the ratio of the former to the latter is as follows:

Provided m 0.07, the ratio is 0.86/0.l4 6. That is, the microwaveradiation has a much higher effectiveness for drying than the hot air.In the other words, the drying time can be shortened by the combinationof hot air blowing and electromagnetic microwave radiation.

In the process of the present invention, it is preferable that thedrying air has a temperature not lower than room temperature, morepreferably, 40 to 100C, particularly, 40 to 60C. The hot air is preparedby using the conventional heater such as a gas burner, electric heater,steam heater or heat exchanger. Also, it is preferable that the dryingair is blown at a velocity of 0.5 to 5 m/sec, more preferably, 1 to2m/sec. The air can be blown by the conventional blower.

In the process of the present invention, the electromagnetic microwaveradiation can be effected by using the conventional radiation device,for example, vacuum-tube type separately excited and self-oscillatorsfor radiating microwaves of very high frequency of 30 to 300 MHz, andmagnetron oscillator for generating ultra microwaves of an ultra highfrequency of 300 to 3,000 MHz. The drying capacity of the microwaveradiation device depends on the output power thereof. The necessarydrying capacity can be obtained by selecting an oscillator suitable forthe purpose and adjusting voltage of an electric source device to apertinent value. For example, the oscillators of 2,450 MHz and 5 KWoutput, or of 915 MHz and 20 KW output, can be utilized for the processof the present invention. In the process of the present invention, theradiation of the electromagnetic microwaves may be begun at a stageprior to, simultaneously with or after the beginning stage of blowingthe drying air. However, it is important that the radiation is effectedat at least the later stage of the hot air drying. Generally, theradiation of the microwaves cause a rapid elevation of temperaturequiteuniformly throughout the porous material, i.e., the outermost andinner portions. This rapid elevation of temperature may result in achange of quality of the porous material. Therefore, it is preferablethat the radiation is begun at a stage simultaneous to or after thebeginning stage of blowing the drying air. By this process, the vaporaround the surface of the porous material is blown away by the dryingair so as to promote the migration of the volatile liquid from the innerportion to the outermost portion. The migrated volatile liquid israpidly evaporated at the surface of the porous material whilemaintaining the partial pressure of the vapor from the volatile liquidat a low level. Almost all the radiation energy of the electromagneticmicrowave is converted to latent heat for evaporation of the volatileliquid, the drying of the porous material can be successively carriedout at an approximately constant rate. The drying capacity of theradiation may be larger than that of the drying air. However, the largeenergy of the radiation may result in fusion, dissolution ordeterioration in quality of the porous material such as a polyvinylacetal porous article. Therefore, it is preferable that the dryingcapacity of the radiation is the same as or smaller than that of thedrying air. The radiation of the microwaves may be carried outcontinuously or periodically. In periodical radiation, it is preferablethat total capacity of the microwave radiation for drying is smallerthan that of the drying air.

In the process of the present invention, the drying may be applied tothe porous material in a stationary or dynamic state. Also, the porousmaterial may move continuously or intermittently through a dryingapparams.

The apparatus of the present invention comprises a closed drying chamberto contain the porous material to be dried, means for blowing drying airinto the drying chamber and means for imparting electromagneticmicrowaves to the porous material. The drying chamber must be surroundedby a material capable of shielding the electromagnetic microwaves andultra microwaves. The shielding material can prevent the leakage of themicrowaves from the drying chamber by reflecting the microwaves so thatall the radiation is absorbed by the porous material charged into thedrying chamber. The shielding material is selected from metal plate andpunched metal plate or net having pores of a size very much smaller thanthe wave length of the imparted microwaves. The metal plate and punchedmetal plate or net may consist of iron, preferably of a non-magnetic andhigh electroconductive metal such as copper and aluminium. The dryingchamber may be formed by the above-stated metal plates themselves. Also,the drying chamber may be formed by an inner wall made of the punchedmetal plate or metal net and an outside wall made of wood or metallicmaterial.

FIG. 2 shows an apparatus of the present invention for drying a porousarticle in a stationary state. Referring to FIG. 2, several porousarticles 1 are charged into a drying chamber 2 by opening a door 3 andhung using hangers 4 and rings 5 in a stationary state. A magnetronoscillator 6 is disposed outside the drying chamber 2, and the output ofthe oscillator is controlled by an electric source device (not shown inthe drawing). The oscillator 6 is connected to the drying chamber 2through a waveguide 7 and a radiation opening 8. The electromagneticmicrowaves oscillated by the oscillator 6 are conduced to the opening 8through the waveguide 7 and directed into the drying chamber 2 throughthe opening 8. A motor 9, a fan 10, engaged with the motor 9, and aheater 11, located above the fan 10, are arranged in a lower chamber 2aformed beneath the drying chamber 2. The lower chamber 2a is connectedto the drying chamber 2 through a punched plate or net 12 and providedwith partitions 2b for forming paths 2c, through which drying aircirculates, a thermometer 13, inserted into the path 2c, and a punchedplate or net 16 for connecting the lower chamber 2a to atmosphere. Ablower 15 is disposed beneath the oscillator 6 to cool the oscillator 6.A cooling air flow generated by the blow 15 passes through theoscillator 6 while cooling it and, thereafter, is introduced into thedrying chamber 2 through the waveguide 7 and the opening 8.

When the fan 10 is rotated by the motor 9, air in the lower chamber 2ais circulated, as drying air flow, through the heater 11, in which thedrying air is heated to a desired temperature, the punched plate or net12,

the drying chamber 21 and the paths 20, along the circulation paths asshown by arrows 14. A part of the drying air is exhausted into theatmosphere through the punched plate or net 16. The oscillator 6 isactuated, preferably, at at least the later stage of the drying process.In this case, the oscillator preferably has a drying capacity not higherthan that of the drying air flowing through the drying chamber. Thedrying operation of the apparatus of FIG. 2 is carried out in thecondition that the drying chamber 2 is closed so as to prevent leakageof the microwaves.

FIG. 3 shows an apparatus of the present invention for continuouslydrying the porous material in a dynamic state. Referring to FIG. 3, aporous material 17 is charged into a drying chamber 18 through anentrance 27 at a predetermined velocity. The drying chamber 18 containsa plurality of ducts 19 having nozzles 20 opening to the porous material17 and connected to a drying air supply (not shown in the drawing). Thedrying air may be conditioned at a predetermined temperature by a heatexchanger (not shown) and at a predetermined pressure by a blower (notshown). The porous material 17 is supported by rotatable rollers 21while advancing through the drying chamber.

A magnetron oscillator 22 is disposed above the drying chamber 18 andconnected to the inside of the dry- 1 ing chamber 18 through a waveguide23 and a radiation '1 opening 24. The opening 24 is located close to theexit 7 28 so as to direct the microwave to the porous material at alater stage of drying. The opening 24 may be located close to theentrance 27 or at a middle portion of the drying chamber 18. A rotatablescattering plate 26, which is rotated by a motor 25, is located in frontof the opening 24 so as to uniformly scatter the microwave directed intothe inside of the drying chamber l8 through the opening 24. The entrance27 and exit 28 and advances along the path formed by the rotatablerollers 21, while being subjected to the'blown drying air ejectedthrough the nozzles 20 of the ducts l9.'At a later stage of the advance,the porous material is exposed to the radiation of microwaves which areregenerated by the oscillator 22, conduced through the waveguide 23 andthe opening 24 and scattered by the rotatable plate 26. After completingthe drying, the porous material is continuously delivered through theexit 28.

The drying may be carried out periodically using the apparatus as shownin FIGS. 4, 5 and 6. In FIGS. 4 and 5, a floor conveyor 41 advancesthrough a drying chamber 40 along a closed path (not shown). Theconveyor 41 is provided with a plurality of joint apertures (not shown)for fastening therein joint members 43 of carriages 42a, 42b and 42c.The carriages 42a, 42b and 420 containing a large amount of porousmaterials are drawn by the conveyer 41 along the closed path. The jointmembers 43 of the carriages 42a, 42b and 43b can be automaticallyremoved from the joint apertures of the conveyor 41 when a stoppingdevice 44, disposed at a suitable position of the closed path, isactuated so as to stop the advance of the carriages, or the carriagescontact the foregoing carriage. Also, the joint members 43 can beautomatically fastened with the joint apertures when the stopping device44 releases its actuation so as to advance the carriages along theclosed path or a carriage is separated from the foregoing carriages.

The drying chamber 40 is provided with three pairs of microwaveradiation devices 45a, 45b and 450 located at positions A, B and C,respectively. When the carriage 42a stops at a position A by actuatingthe stopping device 44, the carriages 42b and 42c following the carriage42a automatically stop at positions B and C. Each pair of the microradiation devices 45a, 45b, 45c, impart microwaves horizontally to theporous materials contained in the carriages42a, 42b and 42c,respectively, at right angle to the direction of the advance of thecarriages.

The microwave radiation devices 45a, 45b and 456 have a plurality ofradiation openings 46a, 46b and 460 facing the block portions of thecarriages, respectively, and extending vertically. The radiationopenings 46a. 46b and 460 are located in a distribution wherein all theporous material in each carriage can be imparted uniform radiation ofthe microwaves.

For example, referring to FIG. 5, the carriage is subjected to threeradiations of the microwaves at positions C, B and A, successively. Inthis case, the radiation openings are located so that the carriage isimparted, at position C, the radiation at the hatched portion thereof,at position B at another hatched portions thereof at which the carriagehas not been imparted the radiation at position C, and at position A ata further hatched portion thereof at which the carriage has not beenexposed to the radiation at either positions C and B. By theabove-mentioned arrangement of the radiation openings, the porousmaterial in the carriage can be uniformly dried. l

When the carriages 42a, 42b and 42c stop in the drying chamber 40, doors47 and 48 close the drying chamber so as to prevent leakage of themicrowaves, and a supplementary carriage 49- stops at a predeterminedposition D outside the drying chamber 40 by action of a stop device 50.After the radiation is completed, the doors 47 and 48 are opened, thecarriage 42a leaves the drying chamber 40 and the carriage 49 entersinto the drying chamber 40.

The drying chamber 40 is provided, at positions A, B and C, with threepairs of blowers 51a, 51b and 51c for drying air. When the threecarriages stop in the drying chamber by the action of the stop deviceand the doors close, the blowers are automatically actuated. That is,the blowers, microwave radiation devices, doors and stop device arecontrolled on and off by a control device (not shown in the drawings) inaccordance with a predetermined operation program. The above-statedoperations are repeated in accordance with the program.

Referring to FIG. 6, the openings of the radiation devices (not shown inthe drawing) extend horizontally and are located in positions facing thehatched portions of the carriages 42a, 42b and 420 in the drawing. Theporous material contained in each carriage can beuniformly imparted theradiation at positions C, B and A, while being dried by drying airgenerated by blowers 51a, 51b and 510. That is, the carriage isimparted, at position C, the radiation of the microwaves at the hatchedportion thereof, at position C at another hatched portion thereof atwhich the carriage has not been imparted the radiation at position C,and at position A at a further hatched portion thereof at which thecarriage has not been imparted the radiation at both positions C and B.

The periodical apparatus as shown in FIGS. 4 to 6 is valuable for dryinga porous material having a relatively large volume or an accumulation ofnumerous amounts of small porous materials both of which need a longtime to complete drying.

FIGS. 7 and 8 show another embodiment of the apparatus of the presentinvention for periodically drying the porous material.

Referring to FIGS. 7 and 8, the carriages 42a, 42b and 42c each having amechanism for circulating the porous material to be dried therewithin, aframe 75 supporting the circulating mechanism and wheels 76 carrying theframe 75 along a closed path formed by the floor conveyer 41.

Referring to FIG. 7, the carriage 42a, 42b and 420 have rotatable shafts71a, 71b and 71c disposed on the frames 75 and have electromagneticclutch joints 73a, 73b and 730 provided at their ends, respectively. Thedrying chamber 40 is provided with motors 72a, 72b and 720 haveelectromagnetic clutch joints 74a, 74b and 74c provided at ends of therotating shafts of the motors, respectively. Further the drying chamber40 has electromagnetic microwave oscillators 45a, 45b and 450 andblowers 51a, 51b and 51c located at positions A, B and C. The carriagescan be engaged with and removed from the floor conveyer 41 in the samemanner as that of the apparatus of FIGS. 4 through 6.

Referring to FIG. 8, the shaft 71a is fastened on the uppermost part ofthe frame 75 and the clutch joint 73a is fixed to an end of the shaft71a. A pair of chain wheels 77 are fastened at both the ends of theshaft 71a. In the same way, other shafts 78 through 84 are disposedparallel to the shaft 71a as shown in the drawing, and each having apair of chain wheels 86 through 91. The chain wheels 77 and 86, 87 and88, 89 and 90, and 91 and 92 are respectively connected to each other bychains 85. The rotatable shafts 78, 79, 81 and 84 have a pair of theother chain wheels 92 through 96 fastened at both the end portionsthereof, respectively. The chain wheels 93 through 96 are connected witha pair of chains 97. A plurality of tie bars 99 are bridged between thechains 97, and a plurality of hangers 98 for the porous materials 1 areattached to the tie bars 99.

In FIGS. 7 and 8, when the carriages 42a, 42b and 42c enter the dryingchamber 40 and stop at the positions A, B and C, respectively, thejoints 73a, 73b and 73c engage with the joints 74a, 74b and 74c so as toconnect the shafts 71a, 71b and 71c to the motors 72a, 72b and 720, andthen the motors are actuated. By the rotation of the motor, thecirculation mechanism of each carriages is driven so as to circulate theporous materials hung on the tie bars 99 along the path of the chain 97.In the above-mentioned system, during the circulation of the porousmaterials, the microwave drying and the air drying are effected for theporous materials. By the circulation in the apparatus as shown in FIGS.7 and 8, the porous materials are uniformly dried.

The periodical apparatus as shown in FIGS. 7 and 8 is valuable forsimultaneously drying numerous porous materials each having a relativelysmall volume.

The apparatus of the present invention may have a device for preventingleakage of microwaves through the path of the conveyer chain, as shownin FIG. 9. If

the leaked microwaves are absorbed by the conveyer chain the absorptionresults in undesirable over-heat of the conveyer chain.

In FIG. 9, the carriage 42a has a bottom plate 100 effective forpreventing leakage of the microwaves imparted to the porous materials 1in the carriage 42a, through the bottom plate 100. In order to preventthe leakage of the microwaves through the space between the lowersurface of the bottom plate 100 and the upper surface of the bottom ofthe drying chamber 40 and a duct 101 containing the conveyer chain 41, ashield plate 103 extends from the lower surface of the bottom plate 100toward the bottom of the drying chamber 40. Also, the shield plate 103covers the joint member 43. The shield plate 103 can reflect themicrowaves directed to the conveyer chain 41 in the duct 101. Thereflected microwaves are further repeatedly reflected on the insidesurface of the drying chamber 40 so as to result in complete absorptionthereof by the porous material in the carriage. That is, the shieldplate 103 is effective for preventing the overheating of the conveyerchain.

Also, the leakage of the microwaves can be avoided by filling the gutter101, containing the conveyer chain 41, with water or other liquidcapable of absorbing the microwaves even-if no shield plate is used. Theduct 101 may be partly filled with water as shown in FIG. 10. Referringto FIG. 10, a part 102 of the duct 101 in a position A in the dryingchamber, at which the carriage is exposed to the microwave radiation, isformed at a lower level than that of other parts. Thus the low levelpart 102 of the duct 101 is filled with water to prevent the leakage ofthe microwaves therethrough. The microwaves are attenuated by repeatedlyreflecting against the inside surface of the duct part 102, and absorbedby the water. This is effective for preventing the absorption of themicrowave by the conveyer chain and thus, the over-heat thereof.

The features and advantages of the process and apparatus of the presentinvention are further described by the following examples which are notintended to limit the scope of the present invention.

EXAMPLE I Cylindrical spongy polyvinyl formal articles, each having adiameter of about mm and a length of 250 mm and containing about percentof water based on the weight of the article, were dried using theapparatus as shown in FIG. 2. The drying apparatus has a magnetronoscillator of a maximum output of 600 watts capable of divergingmicrowaves of a frequency of 2,450 MHz, and means for blowing dryingair.

The drying chamber was charged with the spongy articles and hermeticallyclosed. The drying air having a temperature of 75C was circulated at aflow rate of about 1 m/sec at parts around the spongy articles. Theoscillator was actuated at outputs of I00 and 200 watts.

The drying capacity of the drying air was 22 percent /hour and those ofthe microwaves of 100 and 200 watts were 9 and 17 percent /hour,respectively.

When the drying was carried out only by the drying air at 75C, thespongy articles were dried along curve A in FIG. 1. That is, the spongyarticles could reach a moisture content of about 3 percent after dryingfor a long time of about 18 hours. However, in the case where the hotair drying and microwave drying at an output of 100 watts weresimultaneously effected for the spongy articles, the drying wascompleted in about 5 hours. Further, in the case where the microwavedrying at an output of 200 watts was effected simultaneously togetherwith the hot air drying, the spongy articles were completely dried inabout 3 hours.

The drying capacities of the l watt microwaves and 200 watt microwavesare respectively in ratios of 0.4l and 0.78 with respect to the dryingcapacity of the hot air of 75C. In spite of the small drying capacitiesof the 100 and 200 watt microwaves, it was extremely pleasing that thesimultaneous use of the hot air flow with 100 watt microwaves having adrying capacity ratio of 0.41 to that of the hot air flow resulted in ashort drying time of hours, and; further, that the simultaneousutilization of hot air flow with 200 watt microwaves of a dryingcapacity ratio of 0.78 resulted in a very short drying time of 3 hours.

In the case where the 100 watt microwaves were used together with thehot air, the spongy article was raised to a maximum temperature of 77Cin the inner portion thereof. In the case of 200 watt microwaves, themaximum temperature of the inner portion of the spongy articles was 79C.In these cases, the hot air was of a temperature of 75C. In both thecases, in spite of high heat-sensitive property of the polyvinyl formal,the dried spongy articles had a uniform quality.

However, the comparison drying by only hot air of 75C, resulted in alarge difference of about 30 percent in density, which depends on thepore size and porosity, between the outermost and inner portions of thespongy article.

For further comparison, the spongy articles were dried by the 200 wattmicrowaves only for about 3 hours. In this case, the inner parts of thespongy articles rose to a temperature of about 100C and fused. Thisresulted in articlesof no value commercially.

From the results as stated above, it is obvious that the process andapparatusof the present invention is effective for uniformly evaporatingwater within the spongy article at a substantially constant ratethroughout drying stage, and therefore, results in a uniform quality ofthe dried spongy article. I g

In the apparatus of FIG. 2, the hanger disposed. on the ceiling of thedrying chamber may be rotated so as to rotate the spongy articles hungfrom the hangers. This is effective for uniformly exposing the spongyarticles to the microwave radiation. Further, a plurality of radiationopenings may be formed on the ceiling of the drying chamber andconnected to the oscillator through the corresponding waveguidesbranched from a main waveguides. This is effective for uniformlydirecting the microwave into the drying chamber.

EXAMPLE 2 A length of spongy polyvinyl formal sheet having a width of lm and a thickness of 5 mm and containing 100 percent of water based onthe weight of the sheet was continuously dried using the apparatus shownin FIG. 3. The oscillator generated microwaves of a frequency of 2,450MHz at an output of 1 KW. Dryingair of a temperature of 95 to 97C wasblown onto the spongy sheet at a velocity of 1.4 m/sec. The dryingcapacities of the microwave and the hot air were 0.5 percent/min and0.21 percent/min, respectively. Accordingly, the ratio of the dryingcapacity of hot air to that of the microwave was 1 0.42.

The spongy sheet could be dried to a moisture content of 3 percent byadvancing through the drying chamber at a velocity of 4.8 m/min for 5.5minutes.

For comparison, the spongy sheet was dried by the apparatus of FIG. 3without using the oscillation. When advanced at a velocity of l m/minfor 28 minutes through the drying chamber, the spongy sheet was dried toa moisture content of 5 percent based on the weight of the sheet.

That is, the combination of the hot air drying with the microwave dryinghaving a drying capacity of 0.42 times that of the hot air could shortenthe drying time to about one-fifth that of the hot air only, andremarkably increase efficiency of production of the spongy sheet.

In the apparatus as shown in FIG. 3, numerous pieces of the porousmaterial may be successively carried by an endless conveyer made of anorganic material having a small absorption for electromagneticmicrowaves and a high thermal stability, for example, a polyolefin ornatural or synthetic rubber belt, or of non-magnetic metal, for example,a copper or aluminium chain, net or belt. Further, the apparatus mayhave a plurality of microwave radiation openings connected to theoscillator.

The process and apparatus of the present invention have the advantagesas detailed below.

1. Shortened Drying Time The porous material can be quickly dried by theprocess and apparatus of the present invention at a drying velocity ofseveral times that of the conventional process and apparatus using hotair only.

2. Uniform Quality of the Dried Porous Material Even in the case wherean organic substance having a low thermal stability, for example,polyvinyl acetal is rapidly dried, there is no change in quality,because the volatile liquid is evaporated at a constant rate and thereis substantially no elevation of temperature of the inner portion.

3. Low Cost Almost all of the expensive microwave energy is effectivelyconsumed for evaporating the volatile liquid in the porous material, butnot for substantially elevating the temperature of the porous materialitself.

What we claim is: v

p l. A process for uniformly drying a porous material consistingessentially of polyvinyl acetal, which cornprises i. charging a watercontaining porous material con sisting essentially of polyvinyl acetalinto a closed drying chamber;

ii. blowing drying air onto the porous material; and

iii. directing electromagnetic microwaves of a very high or ultra highfrequency having a drying capacity not higher than that of the dryingair onto the porous material at a stage after the beginning stage ofblowing the drying air.

2. A process as set forth in claim 1, wherein the drying air is of atemperature higher than room temperature.

3. A process as set forth in claim 1, wherein the temperature of dryingair is higher than 40C.

4. A process as set forth in claim 1, wherein the dry ing air is blownat a velocity of 0.5 to 5 m/sec.

5. A process as set forth in claim 1, wherein the microwaves have a veryhigh frequency of 30 to 300 MHz.

6. A process as set forth in claim 1, wherein the microwaves have anultra high frequency of 300 to 3,000 MHZ.

7. A process as set forth in claim 1, wherein the porous material is ina dynamic state during drying.

1. A PROCESS FOR UNIFORMLY DRYING A POROUS MATERIAL CONSISTINGESENTIALLY OF POLYVINYL ACETAL, WHICH COMPRISES I. CHARGING A WATERCONTAINING POROUS MATERIAL CONSISTING ESSENTIALLY OF POLYVINYL ACETALINTO A CLOSED DRYING CHAMBER; II. BLOWING DRYING AIR ONTO THE POROUSMATERIAL; AND III. DIRECTING ELECTROMAGNETIC MICROWAVES OF A VERY HIGHOR ULTRA HIGH FREQUENCY HAVING A DRYING CAPACITY NOT HIGHER THAN THAT OFTHE DRYING AIR ONTO THE POROUS MATERIAL AT A STAGE AFTER THE BEGINNINGSTAGE OF BLOWING THE DRYING AIR.
 1. A process for uniformly drying aporous material consisting essentially of polyvinyl acetal, whichcomprises i. charging a water containing porous material consistingessentially of polyvinyl acetal into a closed drying chamber; ii.blowing drying air onto the porous material; and iii. directingelectromagnetic microwaves of a very high or ultra high frequency havinga drying capacity not higher than that of the drying air onto the porousmaterial at a stage after the beginning stage of blowing the drying air.2. A process as set forth in claim 1, wherein the drying air is of atemperature higher than room temperature.
 3. A process as set forth inclaim 1, wherein the temperature of drying air is higher than 40*C.
 4. Aprocess as set forth in claim 1, wherein the drying air is blown at avelocity of 0.5 to 5 m/sec.
 5. A process as set forth in claim 1,wherein the microwaves have a very high frequency of 30 to 300 MHz.
 6. Aprocess as set forth in claim 1, wherein the microwaves have an ultrahigh frequency of 300 to 3,000 MHz.
 7. A process as set forth in claim1, wherein the porous material is in a dynamic state during drying.